Arms scrunch and lose volume when armature moves
Your arm is affected by two extra bones:
Spine.2

Spine.1

That's why your mesh only follows the arm at &quot;one-third speed&quot;; the other two-thirds of the influence comes from these two Spine bones.
If I quickly clean up the Spine bone's influence, the arm moves correctly.

Btw, one quick way to check what bones influence a vert is to select the vert in Edit mode and look in the sidebar under the &quot;Item&quot; tab. You can see a list of all groups the vert is assigned to and their weights. See the extra Spine.1 and Spine.2?